LIMA, May 21 (Reuters) - Dozens of nations are due to sign an international declaration banning cluster bombs in Lima this week, though big producers like the United States, Russia and China are refusing to join in, campaigners said on Monday.

Delegates from around 70 countries are to meet in the Peruvian capital to broaden support for the declaration that was initiated by 46 nations in February in Norway.

Up to 30 additional countries are expected to add their names to the Norway declaration that seeks to ban the bombs next year.

"We expect countries to come to Lima with their sights firmly set on developing a cluster-bomb treaty that makes a difference," said Thomas Nash, coordinator of the Cluster Munition Coalition (CMC), an umbrella group pushing for a ban.

"They cannot back away or turn to half measures and unrealistic technical fixes," he said in a statement.

When cluster bombs explode, they scatter numerous bomblets, each of which can be deadly. Often the bomblets lie dormant, exploding only when they are picked up by unsuspecting civilians, sometimes years after they were dropped.

Among nations expected to sign up in Lima is Laos which, according to organizers, is littered with more unexploded bomblets than any other nation on Earth.

More than 30 years after U.S. airplanes bombed it during the Vietnam War, there are still 10s of millions of cluster bomb duds in the southeast Asian country.

Other countries attending the Lima conference that were absent in Norway include Greece, Guinea-Bissau, Nigeria, Thailand, Saudi Arabia and Yemen, all of which stockpile cluster bombs, the organizers said.

But the world's biggest producers of cluster munitions, the United Sates, Russia and China, have stayed away.

Israel, which was heavily criticized for dropping millions of bomblets in southern Lebanon during its war last year with Hezbollah guerrillas, is also not expected to attend.